The kicker is that Recurring Nightmare includes several early versions of levels later used in Extermination Day. The rest of the reworked Doom episode maps are there, in various states of completion, as well as a number of similarly redesigned Doom II maps. However, as Sergeant_Mark_IV mentions, it contains 50 maps. A demo beta, released on June 19, 2015, allows players to go through the first episode sans the secret "Military Base" level.
This mod sought to rework the original Doom campaign levels to have more detail and believable architecture, while retaining their layout.
Notable because v21RC3 came out a year before Doom Eternal.